3.1.1 Improved Plant Nutrition
Mycorrhizal plants are generally able to tolerate pathogens and compensate for root damage and photosynthate drain by pathogens (Azcon-Aguilar and Barea 1992; Declerck et al. 2002), because AMF enhance host nutrition and overall plant growth. For example, Declerck et al. (2002) found that G. proliferum and a Glomus sp. isolate not only stimulated growth and increased shoot P content of banana in the presence and absence of the root rot fungus Cylindrocladium spathiphylli, but also reduced root damage by the pathogen, indicating direct interactions between the AMF and the pathogen. In contrast, some reports indicate that AMF are capable of biological control activity (Boyetchko and Tewari 1988; Grey et al. 1989; Rempel and Bernier 1990). It is believed that AMF interact equally with host plants, but in fact AMF prefer one host or host cultivar over another, as shown by Grey et al. (1989) who reported that mycorrhizal barley cultivar WI2291 not only exhibited greater control of the barley common root rot pathogen Bipolaris sorokiniana than a mycorrhizal cultivar Harmal, but also produced significantly higher yields. On the other hand, biological control activity is dependent on the AMF species as demonstrated for common root rot of barley by Boyetchko and Tewari (1992). There are suggestions that root colonization by natural AMF communities occurring in field soils has an inverse relationship with B. sorokiniana infection, indicating not only a direct interaction between the AMF and the pathogen, but also an AMF-mediated improvement in host nutrition (Thompson and Wildermuth 1989). In contrast, there are also reports suggesting a lack of interaction between AMF and B. sorokiniana under field conditions (Wani et al. 1991). Interaction between naturally occurring AMF and pathogens or the lack thereof in the field likely depends on the distribution of the organisms particularly under the different crop rotations. Significant reductions in disease severity as a result of AMF colonization and enhanced P uptake followed by modifications in root exudation patterns has also been reported for take-all disease of wheat (Graham and Menge 1982). Improvement in host P nutrition is one of the earliest proposed mechanisms of AMF-mediated pathogen or disease tolerance that is still very pertinent.
Arbuscular fungi are known to enhance plant tolerance to pathogens without excessive yield losses, and in some cases, enhance pathogen inoculum density. This compensation is apparently related to enhanced photosynthetic capacity (Abdalla and Abdel-Fattah 2000; Heike et al. 2001; Karajeh and Al-Raddad 1999) and a delay in senescence caused by the pathogen, which cancels the positive relationship between disease severity and yield loss (Heike et al. 2001). For example, soybean plants grown in the soil infested with M. phaseolina, Rhizoctonia solani, or F. solani exhibited lower shoot and root weight and plant height compared to control plants in soil not infested with the pathogens or with G. mosseae (Zambolin and Schenck 1983). The incidence of infection by the pathogens was not affected by G. mosseae colonization but the mycorrhizal plants were able to tolerate infection of pathogens better than nonmycorrhizal plants. The efficacy and efficiency of AMF in promoting plant growth enables mycorrhizal plants to tolerate pathogens, as demonstrated by Hwang (1988) using alfalfa challenged with P. paroecandrum and Karajeh and Al-Raddad (1999) using olive seedlings. It is unclear whether mycorrhizal alfalfa tolerated P. paroecandrum or if other additional mechanisms were involved. Despite the presence of a pathogen benefits of AMF to susceptible hosts can occur until a pathogen inoculum threshold level, beyond which no AMF-mediated benefits can be realized (Stewart and Pfleger 1977). On the other hand, high tissue P levels in mycorrhizal plants may not only improve vigor and fitness of the plant but also modify pathogen dynamics in the mycorrhizosphere by modifying root exudation (Davis and Menge 1980; 1981; Kaye et al. 1984).
Tolerance of the plant to a pathogen can vary depending on the AMF species and their ability for enhancing host nutrition and growth, although some ineffective AMF species reduce pathogen entry by triggering a defense reaction in plants (Davis and Menge 1981). For example, Matsubara et al. (2000) noted that there were significant differences in the ability of Gi. margarita, G. fasciculatum, G. mosseae, and Glomus sp. R10 to not only enhance asparagus growth but also in their ability to tolerate the severity of violet root rot caused by Helicobasidium mompa. Asparagus seedlings inoculated with Glomus sp. R10 had the lowest incidence of violet root rot. This important fact highlights the care that needs to be exercised in the selection of AMF species for biological control of diseases.
3.1.3 Qualitative and Quantitative Alterations in Pathogen Biomass
Modifications in root exudate composition following changes in host root membrane permeability as a result of AMF colonization (Graham et al. 1981) can enforce changes in the rhizosphere microbial equilibrium (Brejda et al. 1998; Edwards et al. 1998; Kaye et al. 1984; Meyer and Linderman 1986). Changes in the rhizosphere microfloral community can collectively benefit host plants by creating favorable conditions for the proliferation of microflora antagonistic to pathogens such as Phytophthora and Pythium spp. as shown for eucalyptus seedlings by Malajczuk and McComb (1979). Unfavorable conditions induced by AMF colonization resulted in qualitative changes in the mycorrhizosphere that prevented P. cinnamoni sporangial induction in tomato plants (Meyer and Linderman 1986). Proliferation of G. mosseae inside grapevine roots was associated with a significant reduction in replant disease-causing fluorescent pseudomonad inoculum in soil (Waschkies et al. 1994). Promoting AMF diversity that will ensure that at least a component of the AMF community may be active against pathogens can further enhance the benefits of this mechanism.
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